1. Field of the Invention
The present invention related to mobile communication technologies, and more particularly, to a method and apparatus for transmitting a dedicated reference signal.
2. Description of the Related Art
In a Long Term Evolution-Advanced (LTE-A) system, in order to support a higher peak speed, each cell may configure multiple transmitting antennae, e.g. 8 transmitting antennae; at the same time, in order to reduce overhead of pilot signals, a user equipment (UE) demodulates downlink data by using a dedicated reference signal (DRS) sent by a base station. When sending the DRS, the base station respectively sends the DRS for each path of the data, i.e. each data stream; after receiving the DRS, the UE demodulates each path of the data.
In the LTE-A system, data of multiple UEs may be transmitted in the same time-frequency resources by using a multiple users-multiple input multiple output (MU-MIMO) technology; for the MU-MIMO based on the DRS, the base station configures that multiple UEs of the MU-MIMO adopt different DRS patterns, and preferably, the DRS patterns are orthogonal, so that the UEs can obtain better channel estimation performances. FIG. 1 is a schematic diagram illustrating a DRS structure currently adopted by an enhanced downlink transmission technology in a LTE-A version 9. As shown in FIG. 1, in a physical resource block (PRB), on a resource element (RE) adopted for the DRS, multiplexing is performed for two paths of orthogonal DRSs by using a code division multiplexing (CDM) mode, i.e. the length of extended codes of the DRS is 2. Herein, the RE adopted for the DRS is called as a DRS RE, gray blocks shown in FIG. 1 are resources adopted for the DRS.
In order to support more amount of data stream, the DRS REs may be divided into two groups by using a frequency division multiplexing (FDM) mode. As shown in FIG. 2, the DRS REs with the number 1 is the first DRS RE group, and the DRS REs with the number 2 is the second DRS RE group; each DRS RE group adopts the CDM mode so as to support multiplexing of at best four paths of orthogonal DRSs.
The DRS is obtained by multiplying one part of signals by the other part of signals; as shown in FIG. 3, one part of the signals is an orthogonal extended code sequence 301, represented as w(n), the other part of the signals is a scrambling code sequence 302, represented as r(n), thus the DRS may be represented as DRS(n)=w(n)·r(n), herein n is an index of the DRS RE.
In the enhanced downlink transmission of the LTE-A version 9, a method for generating the scrambling code sequence r(n) is
            r      ⁡              (        n        )              =                            1                      2                          ⁢                  (                      1            -                          2              ·                              c                ⁡                                  (                                      2                    ⁢                    n                                    )                                                              )                    +              j        ⁢                  1                      2                          ⁢                  (                      1            -                          2              ·                              c                ⁡                                  (                                                            2                      ⁢                      n                                        +                    1                                    )                                                              )                      ,n=0, 1, . . . , 12NRBmax,DL−1, NRBmax, DL is the maximum of the downlink resource blocks; the pseudo-random sequence c(i) is initialized over again at the beginning of each sub-frame, specifically a initialized value is cinit=(└ns/2┘+1)·(2NIDcell+1)·216+nSCID, herein nSCID ε {0,1} is dynamically configured through a physical downlink control channel (PDCCH) or configured as 0, NIDcell is an identity of a cell. As can be seen, the cinit does not relate to the identity of the dispatched UE, but relates to the identity of the cell to which the dispatched UE belongs, that is, the scrambling code sequence r(n) is a cell-specific sequence, i.e. all the UEs in the same cell use the same scrambling code sequence. When dispatching not more than two UEs, the base station may obtain the orthogonal DRSs by distributing a scrambling code sequence corresponding to the same nSCID and different extended codes, so as to optimize the channel estimation performance. When the base station needs to pellucidly dispatch more than two UEs, an additional DRS is generated by using a cell-specific scrambling code sequence r′(n) corresponding to another nSCID, r′(n) and r(n) are independent random sequences.
Since the scrambling code sequence is the cell-specific sequence, single-cell MU-MIMO is effectively supported, but when multiple-cell MU-MIMO, i.e. a Coordinated Multi-point (COMP) transmission technology needs to be supported, the UEs of different cells respectively adopt cell-specific scrambling code sequences, and thus the scrambling code sequences adopted by the UEs of different cells are different, and the UE can not obtain the scrambling code sequences adopted by the UEs of other cells. In this way, the UE can not know whether other cell sends the DRS at the same PRB, and thus the UE can not restrain DRS interferences between different cells.